COLLIGATIVE PROPERTIES OF SOLUTIONS

1. COLLIGATIVE PROPERTIES OF SOLUTIONS

2. Colligative Properties Colligative properties depend on the number of particles rather than their nature • Lowering of the vapor pressure (Raoult’s law) • Elevation of the boiling point (“ebulioscopy”) • Depression of the freezing point (“cryoscopy”) • Osmotic pressure

3. Total pressure (PA + PB) PAo PBo PA = PAo.xA PB = PBo.xB 0 0.5 1.0 Mole fraction of B (xB) Vapor Pressure of Ideal Solution PAo PBo PA + PB Solvent A Solvent B A + B Raoult’s Law PA = PAo.xA PB = PBo.xB Solvent pressure lowering (PAo-PA)/PAo= xB

4. Tf and Tb • Freezing point depression: • Kf - “cryoscopic constant” or “molar depression constant” {oC/M} • Boiling point elevation: • Kb - “ebulioscopic constant” or “molar elevation constant” {oC/M}

5. p =  Semipermeable membrane Osmotic Pressure van’t Hoff equation:

6. Molecular Weight Determination • Osmotic Pressure: • Cryoscopy and Ebulioscopy:

7. Non-Ideal Solutions Emix 0, Hmix 0 na + nb na nb

8. Non-Ideal Solution Vapor Pressure Henry’s Law: PA = PAo. AxA PB = PBo. AxB “Attracting” components “Repellent” components positive deviation negative deviation PAo PAo PBo PBo 0 0.5 1.0 0 0.5 1.0 Mole fraction of B (xB) Mole fraction of B (xB)

9. Activity and Activity Coefficient • To account for non-ideality of solutions • “activity” is used instead of concentration • in all equations “Activity”:ai = ici Activity coefficient:i